theres the can opener...bout it that i can think of, the guard essentially keeps you from getting submitted, so the person inside the guard should first better their position before going for a submission.

You can get some gi chokes and the ezekiel choke,but in my experience these only work if your much bigger and stronger than the person otherwise they can just use their legs to take the power out of the choke.

When your in someones guard forget about submissions they're not worth the energy you'd expend, just pass the guard.

I would definitely move out of the guard. It is difficult to submit people there. You can pull of a Kimura sometimes. Also with people that have loose guards, if you get their legs apart you can roll into a kneebar.

If you're in your opponent's guard, I believe the only things you should be doing are:
1. Defend the submission
2. Improve your position because once you get the half-guard, the world of subs REALLY opens up for you.